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SP3238EEA-L Datasheet(PDF) 8 Page - Exar Corporation |
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SP3238EEA-L Datasheet(HTML) 8 Page - Exar Corporation |
8 / 20 page 8 Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com SP3238E_100_020111 Receivers The receivers convert +5.0V EIA/TIA-232 levels to TTL or CMOS logic output levels. Re- ceivers are High-Z when the AUTO ON-LINE® circuitry is enabled or when in shutdown. The SP3238E includes an additional non-in- verting receiver with an output R 1OUT. R1OUT is an extra output that remains active and monitors activity while the other receiver outputs are forced into high impedance. This allows a Ring Indicator (RI) signal from a peripheral to be monitored without forward biasing the TTL/CMOS inputs of the other devices connected to the receiver outputs. Since receiver input is usually from a transmis- sion line where long cable lengths and system interference can degrade the signal, the inputs have a typical hysteresis margin of 300mV. This ensures that the receiver is virtually immune to noisy transmission lines. Should an input be left unconnected, an internal 5kΩ pulldown resistor to ground will commit the output of the receiver to a HIGH state. Charge Pump The charge pump is an Exar–patented design (U.S. 5,306,954) and uses a unique approach compared to older less–efficient designs. The charge pump still requires four external Figure 7. Loopback Test Circuit for RS-232 Driver Data Transmission Rates capacitors, but uses a four–phase voltage shifting technique to attain symmetrical 5.5V power supplies. The internal power supply consists of a regulated dual charge pump that provides output voltages 5.5V regardless of the input voltage (V CC) over the +3.0V to +5.5V range. This is important to maintain compli- ant RS-232 levels regardless of power supply fluctuations. Figure 9. Loopback Test results at 250Kbps (All Drivers Fully Loaded) Figure 8. Loopback Test results at 120kbps (All Drivers Fully Loaded) SP3238E TxIN TxOUT C1+ C1- C2+ C2- V+ V- VCC 0.1µF + C2 C5 C1 + + C3 C4 + + LOGIC INPUTS VCC 5kΩ RxIN RxOUT LOGIC OUTPUTS SHUTDOWN GND VCC ONLINE 1000pF 0.1µF 0.1µF 0.1µF 0.1µF |
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